The present invention relates to an aqueous alkaline flood process for recovering oil by injecting an aqueous alkaline solution into a subterranean oil reservoir containing an acidic oil. More particularly, the present invention relates to such a process in which a specified amount of a specified type of preformed cosurfactant material is added to the injected solution in order to solve a problem encountered by the prior alkaline flood processes.
Numerous aqueous alkaline flood processes have been proposed, and various processes involving injecting an aqueous alkaline solution and various preformed surfactants have been described in U.S. patents, such as the following: U.S. Pat. No. 3,777,817 describes injecting an aqueous alkaline solution to satisfy the surfactant adsorption sites on the reservoir rock and then injecting a surfactant-containing aqueous liquid which may also contain alkali. U.S. Pat. Nos. describe injecting aqueous alkaline solutions containing overbased petroleum sulfonate surfactants which are formed by over-neutralizing petroleum hydrocarbon sulfonates. U.S. Pat. Nos. 3,977,470 and 4,004,638 describe injecting an aqueous alkaline solution followed by an aqueous alkaline solution which contains a preformed surfactant which can be substantially any hydrocarbon sulfonate and can be accompanied by polyphosphates and carbonates that enhance the oil displacing efficiency of the process.
Although the prior processes in which preformed surfactants were included in injected aqueous liquid solutions were designed to improve the oil recovery efficiency of similar processes free of the preformed surfactants, a serious problem is present in either type of such prior processes. Whenever an aqueous alkaline solution is injected into an oil reservoir, some or all of the alkali may be consumed by chemical reactions other than the desired reaction of converting petroleum acids to surfactant soaps. For example, multivalent cations dissolved in the water in the reservoir and/or associated with clay or other reservoir rock materials can rapidly consume alkali by forming and precipitating multivalent metal hydroxides or salts. In siliceous reservoirs significant proportions of alkali are consumed by dissolving silicon oxide and by forming alkali metal silicates, etc. Because of such side reactions, if the injected aqueous alkaline solution is dilute, the alkali will propagate slowly through the reservoir rocks. The frontal propagation rate is slow because, as each portion of the injected solution contacts fresh portions of rock, some or all of its alkali content may be consumed by the side reactions. This is repeated over and over, and thus, although the unreactive liquid components of the injected solution may move through the reservoir at the rate corresponding to the rate at which the solution was injected, the movement through the reservoir of the alkali may be much slower. For example, it is disclosed in SPE Paper No. 8995 by Bunge et al that, when an aqueous alkaline solution containing 0.44% sodium hydroxide and 1.0% sodium chloride was flowed through a core of Wilmington sand which initially contained 1.0% calcium chloride solution, more than two pore volumes of the aqueous alkaline solution had to be injected before any of the sodium hydroxide reached the outflow end of the core.
But it is known that, for example, as indicated in U.S. Pat. No. 3,927,716, when an aqueous alkaline solution reacts with an acidic oil, the lowest interfacial tension between the aqueous solution and the oil frequently occurs when the concentrations of the alkali and neutral salt in the aqueous alkaline solution are low and are within a rather narrow range, such as about 0.01 to 0.04% by weight of alkali and 0.5 to 2.0% neutral salt. Thus, a problem remained. The teachings of the prior art do not disclose how to obtain the low interfacial tension required for a good oil recovery while injecting an aqueous alkaline solution containing the high alkali concentration required for a satisfactory rate of alkali propagation within the reservoir.